This invention relates to the detection of gamma radiation. In particular, this invention is an improvement on the directional detector of gamma rays that was the subject of U.S. Pat. No. 4,136,282.
One of the many sources of information that is of use in controlling nuclear reactors is the flux of gamma rays at various locations in the core of the reactor. It is useful to have information about the direction of incidence of gamma rays, of the density of the flux, and of the energies or energy ranges of the gamma rays. While the reactor is operating, the item of maximum interest in controlling the reactor reactor is the power level of operation. It is also true that useful information that is not necessarily related directly to the power level of the reactor can be obtained from information about the flux of gamma rays incident on a particular localized region in the core of the reactor. While most of the localized core monitors have measured properties of neutron fluxes, a small number of sensors deals either directly or incidentally with a flux of gamma rays. These include the following U.S. Patents that are believed to be the most pertinent to the present invention: Nos. 3,603,793; 3,400,289; and 4,091,288, as well as U.S. Pat. No. 4,136,282, of which the present invention is an improvement.
U.S. Pat. No. 3,603,793, entitled "Radiation Detector Solid State Radiation Detector Using an Insulator Between the Emitter and Collector" was issued in 1971 to Warren. While that detector is principally a neutron detector, it does refer to the possibility of generating electrons by an undisclosed process when gamma rays are incident upon an emitter of ytterbium. The emitter of the '793 patent is surrounded by a coaxial insulator that discriminates on an energy basis between prompt electrons and delayed electrons. While the '793 patent refers to gamma rays as a possible cause of electron emission, it appears from the disclosure in that patent that the principal object of that invention is to detect neutrons, with the detection of gamma rays appearing to be almost incidental to the operation of the device.
U.S. Pat. No. 4,091,288, entitled 'Threshold Self-Gamma Detector for Use as a Monitor of High Power in a Nuclear Reactor" was issued in 1978 to LeVert and Cox. It is a lead prism surrounded by a coaxial thin nickel sheet to form a collector. A coaxial polyethelene electron barrier encloses the collector and is separated from the nickel sheet by a vacuum region. The electron barrier is enclosed by a coaxial stainless steel emitter which in turn is enclosed within a lead casing. When the detector is placed in a flux of gamma rays, a measure of the current flow in an external circuit between emitter and collector provides a measure of the power level of the reactor. It should be noted that this detector is isotropic cylindrically in view of the generally cylindrical symmetry that is exhibited in its design.
The U.S. Pat. Nos. 3,400,289 issued in 1968 to Anderson, and 3,940,627 issued in 1976 to Klar, are mentioned here because they were cited in the prosecution of U.S. Pat. No. 4,091,288. They are not believed to be pertinent to the present invention.
Of the patents cited above, only U.S. Pat. No. 4,136,282 provides information about the direction of gamma rays. All the other patents cited describe detectors that are at least cylindrically isotropic and some of those patents are directed to detectors that function primarily to detect neutrons. The '282 patent is directed to an apparatus that has a strip of an electrical conductor of high atomic number backed with a strip of a second electrical conductor of low atomic number. These elements are enclosed within and insulated from an electrical conductor that establishes an electrical ground, maintains a vacuum enclosure, and screens out lowenergy gamma rays.
Most of the detectors that have been used for directional detection of gamma rays, except for the detector of the '282 patent, use collimators of extensive length or elongated detectors that also extend for some distance in the direction of detection or normal to it. Such long detectors or collimators usually require so much space that they are not installed in the cores of nuclear reactors but are instead placed in an external flux. Such long detectors or collimators are also not readily changed in position to change the direction of radiation that they sense. It is desirable to overcome these drawbacks by providing a detector which can be readily placed in or near the core of a nuclear reactor and is small enough that its direction of detection can be readily changed with minimal or no disturbance to the structure of a reactor. Detectors of this type are useful when it is necessary to monitor the performance of a nuclear reactor by obtaining information only about gamma rays that are in a specific energy range and in a specific direction.
It is therefore an object of the present invention to provide an improved directional detector of gamma rays.
It is a further object of the present invention to provide a self-powered directional detector of gamma rays that are in a predetermined energy range.
Other objects will be apparent in the course of a detailed description of the invention.